The present invention relates to a method and apparatus for calculating motion vectors and, in particular, to a motion vector calculation suitable for use in a video de-interlacing system.
Digital television (DTV) signals conforming, for example, to the Advanced Television Systems Committee (ATSC) standard, may have a large number of formats. These formats are typically referenced by the number of horizontal lines in the image and whether each image frame is formed from two fields, each containing alternate lines of the frame (interlaced) or from a single image field containing all of the lines of the frame (progressive). Listed from highest resolution to lowest resolution, the television signal formats defined by the ATSC standard are referenced by the designations, 1080I, 720P, 480P and 480I. In these designations, the number refers to the number of horizontal lines in the image and the letter defines the resulting image as being interlaced (I) or progressive (P).
Television receivers that operate according to the standard set by the National Television Standards Committee (NTSC) display 480 lines of active video information as two interlaced fields and, so, have a resolution of 4801. Most of the existing programming in the United States conforms to the NTSC standard.
ATSC television receivers may support many different types of monitors. An ATSC receiver may, for example, be connected to a multi-sync monitor that can adapt to display whatever signal type is being received. This type of multi-sync monitor is typically referred to as a native mode monitor as it allows each possible type of ATSC signal to be displayed at its intended resolution. Alternatively, ATSC receivers may be purchased that can be connected to a standard NTSC monitor. One such receiver is the TU-DST51 DTV Decoder Set-Top Box manufactured by Panasonic. This receiver converts each ATSC signal type into a 4801 output signal that may be displayed on the NTSC monitor. The Panasonic receiver also supports the other types of monitors, automatically converting the received input signal to the format that is supported by the specified monitor.
It is well known that interlaced video signals have artifacts caused by the interlacing of video fields that occur at two different instants. One such artifact is vertical dot crawl. This artifact occurs at vertical edges in the image, typically at edges between portions of the image having different colors. As the name implies, the vertical dot crawl artifact is seen as a line of dots that seem to move from the bottom to the top of the frame. If the display device supports progressive video signals, these artifacts of interlaced scanning may be removed, or at least mitigated, by converting the interlaced video signal to a progressive video signal before it is displayed.
There are many methods for converting an interlaced video signal to a progressive video signal. Some of these methods are described in a paper by K. Sugiyama et al. entitled xe2x80x9cA Method of De-interlacing with Motion Compensated Interpolation,xe2x80x9d IEEE Transactions on Consumer Electronics, Vol. 45, No. 3, 1999 pp. 611-616. Typically an interlaced video signal is converted to a progressive video signal (i.e. de-interlaced) by inserting interpolated image lines between the existing lines in each image field of the video signal.
In motion compensated de-interlacing systems, a trade off exists between noise performance and small object Tracking. Motion compensation systems that use relatively large block sizes typically have good noise performance, that is to say they do not tend to track noise in the image instead of the underlying image content. These systems, however, do not tend to track small objects well. Thus, if a small object moves in the image sequence from one image to the next, a de-interlacing system that uses a large block size may have difficulty matching the moved object to the same object in the current frame. If, on the other hand, the block size is reduced as in the above referenced article, the small object recognition is improved but the noise performance suffers.
The present invention is embodied in a motion estimation system for finding motion vectors between first and second images. The motion estimation system selects first and second overlapping blocks of pixels from the first image. The motion estimation system then estimates motion between the first and second images to define respective motion vectors for the overlapping blocks. These motion vectors are assigned to respective first and second non-overlapping sub-blocks of pixels within the respective first and second overlapping blocks of pixels.